As the search for offshore oil and gas reservoirs has moved into deeper waters, developers have been forced to search for more cost effective alternatives than the conventional fixed platforms. Beyond about 1200-1600 feet of water, the structural steel necessary for a conventional platform tower makes development uneconomic for all but the largest of reservoirs. The recent drop in oil prices has exacerbated the problem and extended the payback on even these large reservoirs to the point the developers have second thoughts about proceeding with a development project. A less expensive platform support is required.
One of the limiting factors for a fixed platform tower is providing sufficient structural steel to make the tower rigid enough to avoid the problem of resonance. During storms, the waves having the highest energies occur in the five to twenty second frequency interval. In order to avoid the possibility of a cataclysmic failure resulting from harmonic motion of the tower, it is important that the tower be designed to have each of its natural periods fall outside this 5-20 second interval. For a fixed platform, this requires the addition of significant amounts of steel to reinforce the tower to increase its rigidity. Even then, the first natural period will still normally fall in above the 5 second region, putting the structure at risk.
A more recent design alternative has been to make the tower compliant, i.e., to permit the tower to move responsive to the force of the waves and then to return to its initial, or at rest, position. This alternative permits the tower to be designed to have a fundamental (first) natural flexural period that exceeds 20 seconds, reducing the hazard of resonance. Since the platform tower can be less rigid, the structural steel required can be reduced, producing a potential cost savings. However, the compliant designs proposed to date each have a feature that offsets the potential savings, e.g., guy wire systems, buoyancy tanks, a system of elongated load-bearing piles, a complex pivot arrangement, etc.
The present invention is directed to a cost-effective alternative enabling hydrocarbon production in water depths in excess of 2000 feet (610 m) up to depths of 4000 feet (1220 m) and, possibly, even greater. The tower is comprised of at least two stacked, articulated sections that behave as a fixed platform in quiescent conditions, i.e., the weight of the upper sections is transmitted through structural supports in the lower and base sections to the ocean floor. The base section can be a gravity base or a steel base that is piled to the ocean floor. In the event of a storm with high energy wave and wind forces, the tower behaves as a compliant tower, moving with those forces and being restored to its rest position by a plurality of tension elements that are increasingly tensioned by the compliant motion; the greater the movement, the larger the restorative force. The tower is designed such that all of its natural periods are outside the critical 5-20 second interval. The tower sections are each interconnected by a resilient joint means and, if there are more than two tower sections, each of the subsequent sections is directly interconnected to the base or to one of the other lower sections (depending on flexibility requirements) by its own set of restoring tension elements.
Various other characteristics, features and advantages of the present invention will become apparent after a reading of the following detailed description.